Technical Field
The present disclosure relates to a lock-up device, and particularly to a lock-up device for a torque converter, which is configured to transmit a torque from a front cover to a transmission-side member through a turbine.
Background Information
Torque converters are often equipped with a lock-up device for directly transmitting a torque from a front cover to a turbine. Such a lock-up device includes a piston, an input-side plate, a plurality of torsion springs, and an output-side plate. The piston is capable of being coupled by friction to the front cover. The input-side plate is fixed to the piston. The torsion springs are supported by the input-side plate. The output-side plate is elastically coupled through the torsion springs to the piston and the input-side plate in a rotational direction. The output-side plate is fixed to the turbine.
Additionally, as described in Japan Laid-open Patent Application Publication No. 2013-117282, there has been also proposed a type of lock-up device that a plurality of clutch plates are used to increase its clutch capacity.
In the lock-up device described in Japan Laid-open Patent Application Publication No. 2013-117282, an oil chamber for activating the piston is defined between the piston and the turbine. To define the oil chamber, a boss made in the shape of a sleeve is fixed to the inner peripheral part of the front cover, and the piston is supported by the outer peripheral surface of the boss while being movable in an axial direction. Additionally, the boss is provided with oil channels for leading hydraulic oil to the oil chamber.
In the lock-up device described in Japan Laid-open Patent Application Publication No. 2013-117282, each of the oil channels for leading the hydraulic oil to the oil chamber is made in the shape of a hole penetrating the boss from its inner peripheral side to its outer peripheral side. In this construction, the amount of hydraulic oil flowing into the oil chamber cannot be limited. Hence, the piston is activated with a great force, and shock occurs when the clutch plates are pressed in contact with each other, in other words, when a clutch-on state is made.
On the other hand, increase in size of each clutch plate is required to increase the clutch capacity of the lock-up device. Additionally, increase in outer diameter of the boss is required with increase in size of each clutch plate. In this situation, each of the oil channels (holes) provided in the boss inevitably has a quite large length with respect to the diameter of each hole. This construction makes it difficult to process the oil channels.